a) Field of the Invention
The invention is directed to a device for rapid and highly precise detection of single defects.
b) Description of the Related Art
A device for profile measurement of surfaces which has a polychromatic light source is known from GB 2144537. The illumination light is split longitudinally in spectra and focused on an object, wherein a determined wavelength corresponds to every focal point. The light reflected by the object travels via a beam splitter to a dispersive element and is focussed by the latter on a photodiode array. The strongest signal is determined by the readout of the photodiode array and is related to the surface of the object.
WO 88/10406 describes a device for the measurement of distances between an optical element with extensive chromatic aberration and an object, likewise for profile measurement, with a structured light source and a spectral-dispersive apparatus and CCD camera.
U.S. Pat. No. 4,965,441 describes a scanning, i.e., point-scanning, confocal arrangement with increased depth resolution, wherein dispersive elements are arranged in the evaluating beam path for wavelength separation.
The principle of a scanning image generation process with an objective lens with a strong chromatic aberration and spectral separation of the light backscattered by the object is also the subject of DE 4419940 A1.
WO 92/01965 describes an arrangement for simultaneous image generation with a moving pinhole in the illumination beam path, wherein the objective can have a strong chromatic aberration and the arrangement is to be used as a profile senor.
WO 95/00871 likewise discloses an arrangement with a moving pinhole and a focusing element with axial chromatism, wherein spectral filters are arranged in front in the evaluating beam path of two cameras. Signal division is effected by pixel to determine wavelength.
Arrangements of the above-mentioned kind are also known from SCANNING, Vol. 14, 1992, pages 145-153.
Further, a system for the generation of a color height image is realized, for example, with the CSM attachment for the Axiotron microscope by the present Applicant. The height profile is inspected by visual evaluation of the color image.
In this connection, a special application is wafer inspection, i.e., the detection of defects on wafers (e.g., settled particles, irregularities in the structure). Defects are visible as chromatically delimited areas. Height differences of less than 0.1 xcexcm can accordingly be differentiated by color. The accessible height range, the color spread, depends on the utilized objective and is, for example, 4 xcexcm in the case of a 50xc3x97 objective.
In automatic wafer inspection, fast detection of defects is necessary. The usual methods used in practice work on the basis of laser scattering or digital image processing. Accordingly, at the present time, defects in the range of 0.2 xcexcm can be detected with a typical throughput of 20 wafers (xc3x8=200 mm) per hour. Laser scattering methods are limited to the detection of particles (dirt, dust), while digital image processing can also detect other types of defects such as structural defects or xe2x80x9cpattern defectsxe2x80x9d, as they are called.
As the integration density of electronic circuits increases, fewer detection limits are required. In a 1 GB DRAM, for example, detection of a defect size of 0.1 xcexcm is considered necessary. In methods using digital image processing, the minimum detectable defect size is determined by the resolving capacity and the speed of inspection is determined by the computer capacity. In this connection, a considerable expenditure on electronic computing is required to process the information at a suitable speed. For example, when scanning a wafer with a 300-mm diameter with a 0.3 xcexcm grating, a total of 1012 pixels must be processed in digital image processing.
It is the primary object of the invention to enable a fast yet highly precise detection of wafer defects.
According to the invention, this object is met by the independent claims. Preferred further developments are described in the dependent claims. a confocal microscope arrangement comprising an illumination arrangement for illuminating an object in a raster pattern, first means for generating a first wavelength-selective splitting of the illumination light and second means for generating a second wavelength-selective splitting of the light coming from the object in a parallel manner for a plurality of points of the object. Also included are detection means for deflecting the light distribution generated by the second means.
This color image information has the advantage that defects whose height dimension is less than 0.1 xcexcm can be detected. Further, the optical pre-processing by means of spectral division enables a faster processing of the respective height profile since the height profile of the entire image field is compressed in the spectrum. A highly accurate autofocus which will be described in more detail hereinafter can likewise be realized by the arrangement according to the invention.